Capsule
Adult Black-headed Gulls Chroicocephalus ridibundus brought the HPAI infection to a mixed gull and tern breeding colony, but were less severely affected than adult Common Terns Sterna hirundo.

Aims
To report the impact of a 2023 major HPAI outbreak on populations of Black-headed Gulls and Common Terns at a colony at Shotton, Flintshire (Wales), and make recommendations for dealing with future HPAI impacts on breeding gulls and terns.

Methods
We examined the number of birds affected by HPAI by species and age, tracking the infection through the colony and detailing measures taken to respond to the outbreak.

Results
In total, 236 adult Black-headed Gulls (9% of the colony) and 1226 chicks were found dead, reducing the number of fledglings from 1250 in 2022 to 75–100 in 2023. Compared to the gulls, proportionally more adult Common Terns died: 320 carcasses and 171 nests were counted in 2023, indicating a minimum adult mortality of 40%. Data from the 2024 breeding season implied a 75% reduction of breeding Common Terns. Among the dead terns in 2023, 69% of those previously ringed at Shotton were initially captured in 2014–2019, but after accounting for natural mortality there was no age-related bias in HPAI mortality. The lack of an action plan and decision-making delays resulted in carcasses remaining uncollected for 28 days, after which 378 dead adults and 376 dead chicks were removed.

Conclusion
Black-headed Gulls suffered very high levels of chick mortality but modest adult deaths, which were unlikely to have significantly impacted the colony’s viability. Common Terns were severely impacted by HPAI at the already struggling colony, and 2024 counts underscored the threat to colony’s viability. HPAI outbreaks require a rapid response, and required actions are straightforward for an organized team. Managers should prepare an advance response plan with personnel comfortable with the tasks required.